FIX: Make testing slightly cleaner

This does not fix the problems in `master`, but it should make them easier to diagnose. Ready for review/merge from my end. 
This was doing `np.uint8` subtraction before, which isn't as useful as doing a signed-integer -> abs computation (e.g., having a blank output yielded `blank|circle|blank` image instead of `blank|circle|circle` because of the unsigned int arithmetic